1. Field of the Invention
The present invention relates to a spindle motor structure supported by a hydrodynamic bearing and a hard disk drive employing the same and, more particularly, to a spindle motor structure which can smoothly rotate a spindle motor in a low temperature atmosphere by heating the spindle motor without structural modification of the spindle motor itself, and a hard disk drive employing the same.
2. Description of the Related Art
Hard disk drives (HDDs) are one type of information storage media that record data on a disk rotated by a spindle motor or reproduce data from the disk using a read/write head.
FIG. 1 is an exploded perspective view of a conventional hard disk drive. FIG. 2 is a cross-sectional view showing a part of the hard disk drive of FIG. 1. Referring to FIGS. 1 and 2, a hard disk drive 10 includes a base 11 and a cover member 12 coupled to the base 11. A spindle motor 30 for rotating a disk 20 and an actuator 40 for moving a read/write head 44 to a desired position on the disk 20 are installed on the base 11.
The actuator 40 includes a swing arm 42 rotatably coupled to an actuator pivot 41 installed on the base 11, a suspension 43 installed at an end portion of the swing arm 42 and supporting the head 44 to be elastically biased toward the disk 20, and a voice coil motor 50 for rotating the swing arm 42.
A printed circuit board 70 is coupled to a lower surface of the base 11 with an insulation plate 60 interposed therebetween. Semiconductor chips 71 and various circuit elements 73 to operate the spindle motor 30, the read/write head 44, and the actuator 40 are mounted on the printed circuit board 70.
The spindle motor 30 includes a stator 31 fixed to the base 11, a rotor 33 rotatably installed with respect to the stator 31 and for fixing the disk 20 thereon, a hydrodynamic bearing 35 provided between the stator 31 and the rotor 33 and supporting the rotor 33 to be capable of rotating, and a magnetic driving portion 39 for providing a rotational driving force to the rotor 33.
The hydrodynamic bearing 35 includes a journal bearing portion 36 supporting a rotation shaft 33a of the rotor 33 in a radial direction, a thrust bearing portion 37 supporting the rotation shaft 33a in an axial direction, and fluid 38 injected into the journal bearing portion 36 and the thrust bearing portion 37. Thus, the hydrodynamic bearing 35 generates dynamic pressure as the fluid 38 injected between the journal and thrust bearing portions 36 and 37 and the rotation shaft 33a forms an oil film, when the disk 20 is rotated by the magnetic driving portion 39. Thus, the hydrodynamic bearing 35 can reduce a frictional load by supporting the rotation shaft 33a to be capable of rotating in a state in which the stator 31 and the rotor 33 do not contact each other. In the meantime, the performance of the spindle motor with the hydrodynamic bearing configured as above may be deteriorated at low temperature.
FIG. 3 is a graph showing a change in friction torque according to a change in the temperature of the hydrodynamic bearing. Referring to FIG. 3, in a hard disk drive employing a hydrodynamic bearing, when the ambient temperature is about 15° C. or more, the friction torque of the thrust bearing portion is about 1.5×10−4 [Nm] or less while the friction torque of the journal bearing portion is about 0.5×10−4 [Nm] or less, which do not generate any special problems. However, when the ambient temperature goes down under 5° C., since viscosity of the fluid increases, the friction torque between each of the journal and thrust bearing portions and the rotation shaft increases. For example, in reviewing the change in the frictional torque according to the change in the temperature of the thrust bearing portion, the frictional torque is about 3.1×10−4 [Nm] at −5° C. while, at −10° C., the friction torque drastically increases to about 4.2×10−4 [Nm]. That is, it can be seen that the amount ΔTf of change in the friction torque according to the difference of 5 degrees is about 1.1×10−4 [Nm] which is very large.
Thus, according to the above result, the hard disk drive employing the hydrodynamic bearing has problems that driving current needed to rotate the spindle motor increases at a temperature below zero, in particular, under −5° C., and normal rotation driving is not possible within a predetermined time.
To solve these problems, Japanese Patent Laid-Open Publication No. 5-166291 (published on Jul. 2, 1993 and entitled “Spindle Motor of Magnetic Disk Apparatus”) discloses a method of increasing a rotation speed of the spindle motor by decreasing viscosity of the fluid by heating the hard disk drive. The disclosed apparatus has a heater in the spindle motor so that the viscosity of a lubrication agent is decreased by heating the spindle motor using the heater in a low temperature environment.
However, since the disclosed apparatus has a structure of including the heater inside the spindle motor, the structure of the spindle motor is complicated and costs therefore increase.
Also, in configuring the hard disk drive employing the hydrodynamic bearing, its performance is maximized only when the spindle motor having the above-described complicated heater structure is employed. When the spindle motor which does not include the heater structure as shown in FIG. 2 is employed, the problem that the driving of the spindle motor is still difficult due to the increase in the friction torque in a low temperature environment.